This invention relates to textile manufacturing. More specifically, this invention relates to quality control for stitching of textile articles.
Large aircraft structures such as wing covers are now being fabricated from textile composites. The textile composites are attractive because of their potential for lowering the cost of fabricating the large aircraft structures. Cutting pieces of fabric and stitching the fabric pieces together have the potential of being less expensive then cutting sheets of aluminum, drilling holes in the aluminum sheets, removing excess metal and assembling metal fasteners.
The wing cover can be made from a carbon-fiber textile composite. Sheets of knitted carbon-fiber fabric are cut out into pieces having specified sizes and shapes. Fabric pieces having the size and shape of a wing are laid out first. Several of these pieces are stacked to form the wing cover. Additional pieces are stacked to provide added strength in high stress areas. After the fabric pieces are arranged in their proper positions, the pieces are stitched together to form a wing preform. Secondary details such as spar caps, stringers and intercostals are then stitched onto the wing preform. Such a wing preform might have a thickness varying between 0.05 inches and 1.5 inches. The wing preform is quite large, and its surface is very complex, usually a compound contoured three-dimensional surface.
The wing preform is transferred to an outer mold line tool that has the shape of an aircraft wing. Prior to the transfer, a surface of the outer mold line tool is covered with a congealed epoxy-resin. The tool and the stitched preform are placed in an autoclave. Under high pressure and temperature, the resin is infused into the stitched preform and cured. Resulting is a cured wing cover that is ready for assembly into a final wing structure.
For textile composite technology to be successful, two barriers must be overcome: cost and damage tolerance. Damage tolerance appears to have been hurdled. Closely-spaced stitches on the wing preform provide sufficient damage tolerance because the stitches provide a third continuous column of material.
Cost continues to be the problem. Although the textile composites are less expensive than aluminum, and textile manufacturing techniques are old and proven, the machines for stitching are slow and unreliable. This problem is especially true for wing preforms because an exceedingly large number of stitches must be made, and they must be made in a contoured, compound three-dimensional surface.
Many hours are spent on quality control. Visual inspections are performed to ensure that the stitches have proper spacing and tension. Loose threads and broken stitches are identified. Irregular-sized holes surrounding the threads are also identified. Too small a hole might suggest an overly tight stitch; too large a hole might suggest a loose stitch.
Moreover, the visual inspection is subjective; its accuracy is dependant upon the attentiveness of the person performing the inspection. Still, for a small structure, visual inspection might be feasible. A few defective stitches could be identified and repaired.
However, quality control by visual inspection is extremely slow, costly and error-laden for a wing preform that might have eight to ten stitches per inch, in rows that might be spaced 0.1 inches to 0.5 inches apart, over a surface that might be longer than forty feet and wider than eight feet. Manually finding defective stitches, keeping track of the locations of the bad stitches, and removing and repairing the bad stitches cannot be done quickly, accurately and cost-effectively.
Based on the foregoing, it can be appreciated that there presently exists a need for quality control that can be performed quickly, accurately and cost-effectively. As will become apparent hereinafter, the present invention fulfills this need.